tools/nolibc: add the nolibc subdir to the common Makefile

The Makefile in tools/ is used to forward options to the makefiles
in the various subdirs. Let's add nolibc there so that it becomes
possible to make tools/nolibc_headers_standalone from the main tree
to simply create a completely usable sysroot.
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/Makefile

@@ -24,6 +24,7 @@ help:
 	@echo '  intel-speed-select     - Intel Speed Select tool'
 	@echo '  kvm_stat               - top-like utility for displaying kvm statistics'
 	@echo '  leds                   - LEDs  tools'
+	@echo '  nolibc                 - nolibc headers testing and installation'
 	@echo '  objtool                - an ELF object analysis tool'
 	@echo '  pci                    - PCI tools'
 	@echo '  perf                   - Linux performance measurement and analysis tool'
@@ -74,6 +75,9 @@ bpf/%: FORCE
 libapi: FORCE
 	$(call descend,lib/api)
 
+nolibc_%: FORCE
+	$(call descend,include/nolibc,$(patsubst nolibc_%,%,$@))
+
 # The perf build does not follow the descend function setup,
 # invoking it via it's own make rule.
 PERF_O   = $(if $(O),$(O)/tools/perf,)